This invention relates to analog signal processing devices, and more particularly, to electronic filtering and mixing devices. This invention further relates to the transceiver and components thereof described and claimed in the following U.S. Patent Applications filed of even date with and assigned to the assignee of the present invention: U.S. Ser. No. 791,611 entitled "A Digitally Transmitting Transceiver" by Edward R. Caudel and William R. Wilson; U.S. Ser. No. 791,629 entitled "A Clarifying Radio Receiver" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,449 entitled "An Automatically Clarifying Radio Receiver" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,254 entitled "A Computer Controlled Radio System" by Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,450 entitled "A Transceiver With Only One Reference Frequency" by Michael J. Cochran; U.S. Ser. No. 791,264 entitled "An Electronic Phase Locked Loop" by Michael J. Cochran; U.S. Ser. No. 791,265 entitled "A Signal Strength Measuring Transceiver" by Edward R. Caudel; U.S. Ser. No. 791,614 entitled " A Charge Transfer Device Radio System" by Michael J. Cochran; U.S. Ser. No. 791,253 entitled "A Transceiver Capable of Sensing A Clear Channel" by Jerry D. Merryman, Michael J. Cochran and Edward R. Caudel; U.S. Ser. No. 791,256 entitled "A Highly Selective Programmable Filter Module" by Michael J. Cochran and Edward R. Caudel; and U.S. Ser. No. 791,616 entitled "A Dual Processor Transceiver" by Edward R. Caudel, William R. Wilson and Thomas E. Merrow. Such copending patent applications are hereby incorporated herein by reference. Filtering devices receive electronic input signals comprised of a plurality of non-overlapping frequency bands, and filter a selected band from the plurality. Mixing devices receive frequency signals of one frequency spectrum, and generate output signals having a frequency spectrum proportional to the input signals, but shifted in frequency.
The invention herein described is called a highly selective programmable filter module because it has passbands whose width and center frequency are adjustable in very small increments. Applications for this type of filtering capability are very broad. Typical uses include the processing of signals in radio systems, television receivers, and CB transceivers. As an example of an application for the variable bandwidth feature, some radio receivers demodulate both amplitude modulated signals (AM) and single sideband signals (SSB). The bandwidth of an AM signal is approximately twice the bandwidth of an SSB signal. Therefore, during one time interval, such receivers require a filter having a bandwidth of an AM signal; and alternatively, during another time interval, require a filter having a bandwidth of an SSB signal. The invention herein described has a bandwidth which is selectively adjustable to the width of an AM signal or an SSB signal.
In the above example, the passband width of the invention would be varied by a factor of approximately 2:1. Alternatively, the invention herein described provides passbands with widths which are selectable in much smaller increments. As an example, the width of a 10-kHz bandpass may be increased or decreased in increments of only 5 Hz.
As mentioned above, the invention herein described also has passbands whose center frequency is adjustable in very small frequency increments. This capability may be utilized in a CB transceiver, for example, to filter one band of signals from a plurality of non-overlapping frequency bands. In this application, the center frequency of the filter's passband is adjusted in increments equal to the spacing between adjacent channels. Typically, single sideband channels are separated by approximately 5 kHz; whereas, AM channels are separated by approximately 10 kHz. The invention herein described is capable of performing frequency band shifting for filtering both AM and SSB channels. Additionally, the invention herein described is capable of frequency shifting its center frequency in increments much smaller than adjacent channel spacing. For example, a passband of approximately 5-kHz width may be shifted in increments of 10 Hz by the invention herein described.
In addition to performing a filtering operation, the present invention performs a mixing operation; and, the frequency at which the signals are mixed is also adjustable by very small frequency increments. This capability, in combination with the capability to vary the center frequency of a passband by small increments, may be utilized to perform a clarifying function, as an example. The clarifying function is performed on single sideband signals. Such signals are difficult to demodulate because a sideband may lie anywhere within its assigned frequency channel. As a result, audible tones which are produced by demodulating a sideband channel have either higher frequency components or lower frequency components than should be present dependent upon whether the sideband lies in the upper portion or the lower portion of its assigned channel, respectively. The present invention may be utilized to clarify the resulting audible sound by adjusting the center frequency of its passband to be precisely aligned with the sideband signals regardless of where they lie within a channel. As a result of this precise alignment, the single sideband will be filtered and mixed by a frequency which will compensate for any misalignment of the sideband in the channel.
The above application of the invention are herein given only as examples. The invention has a multitude of applications. and can be applied wherever the bandwidth of a filter is required to be highly selectable, the center frequency of a filter is required to be highly adjustable, or a mixing device is required for shifting an input signal by a highly selectable frequency.
Accordingly, it is one object of the invention to provide an improved electronic filtering device.
It is another object of the invention to provide a filter having passbands with a highly selectable bandwidth.
Another object of the invention is to provide a filter having passbands with a center frequency which are adjustable by fine increments.
Another object of the invention is to provide a mixing device for frequency shifting an input signal by a finely adjustable frequency.
Still another object of the invention is to provide a filter module having passbands and a center frequency which are proportional to a variable reference frequency times the term (N1/N2) N1 and N2 are arbitrary integers.